Humans and many animals have binocular vision provided by two eyes which look in the same direction. Two parallel aligned but spaced eyes deliver two slightly different images of the same scene. This is due to the 4 to 6 cm separation between the eyes, which makes each eye have a slightly different viewpoint. The images from these two different viewpoints are sent to the brain, and this difference, which is termed parallax, is interpreted as depth. The nearer an object is to the eyes, the greater the difference between the two images. From the difference between the two images, the brain can display an impression of depth.
Stereoscopic image displays, such as stereoscopic projection for example, are based on the same principle: two slightly different images, one for the left eye and one for the right eye, are displayed by some means, e.g. projected onto a screen. A corresponding image modulating system enables the left eye to see only the left eye image, and the right eye to see only the right eye image.
There are at least four types of electronic stereo-3D-devices:    a) devices which produce two different images which are so arranged that the left and right eye can only see the relevant image, such as Helmet Mounted Devices (HMD), virtual reality (VR) headsets, or autostereo displays    b) devices which project a single virtual image at a distance which is viewed by both eyes in a normal way, such as Head-Up Displays (HUD), and    c) viewing glasses which influence the way each eye sees the relevant image on the display. These glasses may have liquid crystal-shutter glasses or polarizers or include color filters such as red and green filters.    d) holographic displays, which enable 3D viewing with the naked eye, wherein no goggles nor special glasses are required. Images from two projectors are combined into a 3D viewing zone. Using holographic techniques, one, two or more viewing zones can be recorded. A “look around” effect can be achieved with additional projectors.
HMDs are basically wearable monitors. To allow stereoscopic vision, an image is projected onto a beam splitter in front of each eye. For VR headsets a miniature liquid crystal display (LCD) can be placed in front of each eye.
In active stereo, i.e. when at least one projection device is used to alternately project images for the one eye and images for the other eye, active obscuration devices such as shutter glasses or shutter screens are used in conjunction with a normal CRT, digital light processing (DLP), or equivalent monitor or projector. The two images required for stereoscopic vision are displayed in turn. For a fraction of a second the image dedicated to the left eye is displayed, after that the image for the right eye appears for the same duration of time, and so on. The job of the glasses is to prevent the left eye from seeing the image dedicated to the right eye and vice versa. To do this, light is blocked by a shutter. The frequency of the shutter is adapted to the frequency of the displayed images. The blocking can be done by having a filter which alternates between opaque and transparent.
The principle of passive stereo displaying, i.e. when at least one, but most often a plurality—e.g. two—of projection devices are used to each project images intended to be seen by a different eye, may be based on the principle of different light polarizations or of differences in frequency (color). For example, the viewer wears passive obscuration devices, i.e. non-switchable obscuration devices, e.g. a pair of glasses containing two oppositely polarized lenses or filters, one for the left eye and one for the right eye. The light from each of two projected images is polarized differently and can pass only through its corresponding filter. If the images are provided by means of a single projector, the projector alternates the left eye information with the right eye information at double refresh rate. A screen in front of the projector's lenses alternates the polarization of the projected image in such a way that the image of each eye passes only through its corresponding polarizing filter of the pair of passive stereo glasses. If the images are provided by means of two projectors, one projector displays the left eye information and the other display the right eye information, both at a standard refresh rate. A polarizing filter mounted in the optical path of each projector ensures that the correct information passes through its corresponding filter in the pair of passive stereo glasses. If the projectors are LCD projectors, they may be internally polarized, thus eliminating the requirement for external filters. The two-projector approach has the added value over the one-projector approach of providing higher brightness.
Passive obscuration devices have the advantage over active obscuration devices that they are less expensive. 3D animation is often performed for a larger audience, to which obscuration devices are distributed. Obscuration devices often tend to disappear, which is very annoying in case they are rather expensive, but does not really matter in case they are e.g. a cheap pair of glasses with colored plastic filters as the lenses.
In DE-199 24 167 a method is described to produce stereoscopic images, enabling 3D-perception of objects on a screen. For this purpose, filters with discrete transmission bands are used, one in green, one in red and one in blue. When two display or projection systems are equipped with different filters, for example filter A and filter B, which do not have a common transmission range, and the viewer uses glasses with the corresponding filters, for example filter A in front of eye 1 and filter B in front of eye 2, the stereoscopic image emerges. However, it is a disadvantage of the method of DE-199 24 167 that very steep filters are required, which are difficult to implement, and that, due to differences in wavelengths for the colors shown to each eye, color artifacts appear in the projected images.
U.S. Pat. No. 4,692,792 describes a stereoscopic apparatus in which the spectral components of each perspective of the image are supplied in sequence and in such a way that whilst the left eye is receiving one spectral component of one of the perspectives, the right eye receives the other spectral component of the other perspective, and in alternating periods the left eye receives the other spectral component of its perspective whilst the right eye receives the first spectral component of its perspective. Various means for producing the image and for viewing the two perspectives are illustrated. One example is a composite RGB signal, which during one frame comprises e.g. an R signal for a first eye, and G and B signals for a second eye, while it comprises during a subsequent frame an R signal for the second eye and G and B signals for the first eye. The viewer sees a display screen through a viewing means which is driven in synchronism with the RGB signal. The viewing means may comprise two rotating tubular cylinders or discs, each having a first sector which transmits red and blocks blue and green, and a second sector which blocks red and transmits blue and green. The two cylinders or discs are rotated in the same direction so that when the red sector of the cylinder or disc is facing the viewer's left eye, the blue-green transmitting sector of the other cylinder or disc is facing the viewer's right eye.
Stereoscopic image display may be used, a.o. in keyhole surgery, in entertainment applications, such as gaming environments, in training environments such as in flight simulators, ship bridge simulators, air traffic control, mission rehearsal simulators, and in visualization of large data-sets, such as in car design for example.